Ageing is an inevitable process characterised by a progressive deterioration in the function of a number of organs and systems, ultimately reducing the individual's quality of life. Despite its obvious importance, our understanding of the basic age-related changes in cardiac function remains poor. In part because past studies, describing the changes in cardiac function with age, have not always adequately controlled for different lifestyles or superimposed diseases, while using less than complete measures of overall cardiac function and appropriate scaling models. The aim of this thesis was to determine the changes in overall cardiac function associated with healthy ageing, using the comprehensive, non-invasive method of cardiac power output (CPO). After identifying that CPO was not affected by circadian rhythms, 149 sedentary men and women (19-75 years) and 60 active men were investigated. All were free from cardiovascular diseases and medications. The findings indicated that healthy ageing, in all subject populations, was associated with an ~17% decline in CPOrest, ~15% in CPOmax, and ~14% in CR. Age for age, sedentary men had greater CPO values than women, both at rest (~19%) and at maximal exercise (~25%). In addition, cardiac function was greater in the active men, compared with age-matched, sedentary controls, with values ranging from 11-30% greater. It was also found that body composition had a significant impact on the interpretation of CPO data. For example, when scaled allometrically for fat free mass (FFMb), absolute sex-related differences in CPO disappeared, as did the age-related decline in CPOrest. Also, in women no age-related changes in CPOmax were found once normalised to FFMb even though in men, CPOmax and CR still declined with age. Clearly, how these data are normalised relative to body composition is crucial to the interpretation of the effects of ageing or an active lifestyle. These are novel findings and indicate that healthy ageing is associated with reductions in overall cardiac function, as represented by significant declines in CPOmax and CR in men, but not women. In addition, endurance training improved aerobic capacity while attenuating the changes in cardiac function.